Vents are designed to release air, moisture and other volatile gases following the initial melting of polymers. The velocity of air and/or vapors flowing out of these vent-port openings is a function of volume flow rate and the vent open area. The venting screw inside barrel has a decompression zone in the venting position so the melt pressure is almost zero in the venting port. However, there is a solid plug designed to replace the venting valve. In single stage screws, this plug closes the venting port without the decompression zone. In other words, the venting port will have full melt pressure from single stage screw that may reach to 10,000 psi. In known vent closes, the high pressure melt may flow between the vent plug and a saddle provided around the opening, causing melt leakage to occur between the vent opening and the saddle or causing the mechanical failure of the saddle because of high forces from hydraulic pressure.
In order to minimize leakage, in know applications, the saddle may be welded to the barrel with bimetallic liner. This is a difficult and time consuming process to control minimum residual stress and distortion from welding.
It would therefore be beneficial to provide a vent opening for an extrusion barrel in which the saddle has a seal ring which can be safely used as a venting valve for a two-stage screw and a venting plug for a single-stage screw.